In a conventional injection molding machine designed so that a resin heated and fluidized in a heating cylinder is injected and poured into a cavity in a mold under a high pressure, cooled, solidified and cured in the cavity, and then, the mold is opened to remove a molded product, a clamping device is provided to prevent the resin poured into the cavity from being leaked therefrom through matched surfaces of dies. In an electric-powered injection molding machine using a motor as a drive source, the opening and closing of the mold are carried out by a crank mechanism or a ball screw feed mechanism, and the die-clamping is carried out by a clamping cylinder.
In this case, when the die-clamping has been completed, an oil within the clamping cylinder is drained, and the opening of the mold is carried out by the crank mechanism or the ball screw feed mechanism (see Japanese Patent Application Laid-open No. 39223/91).
FIG. 1 is a schematic view of a clamping device for the prior art electric-powered injection molding machine.
Referring to FIG. 1, the injection molding machine includes a stationary platen 1 and a movable platen 2 which are mounted to dies 3a and 3b of a mold, respectively, and a toggle support 4 movable on a base 5. The movable platen 2 is moved by a single-toggle mechanism 6 disposed between the toggle support 4 and the movable platen 2, thereby performing the opening and closing of the mold.
Four parallel tie bars 7 are disposed at four corners of the toggle support and each slidably passes through holes 1a and 2a defined in the stationary and movable platens 1 and 2. Each of the tie bars 7 is fixed at one end thereof to the toggle support 4 and at the other end to a piston 9 in a clamping cylinder 8 mounted to the stationary platen 1. An oil chamber adjacent a rod of the clamping cylinder 8, i.e., adjacent the tie bar 7 is in communication with a small-sized hydraulic pressure generating unit 11.
The single-toggle mechanism 6 includes a bracket 12 fixed to the toggle support 4, a first link 13 pivotally supported on the bracket 12, a second link 14 pivotally mounted to the first link 13 through a link pin 18, and a bracket 15 rotatably supporting the second link 14 and fixed to the movable platen 2. The first link 13 has a shaft 16 connected to a drive shaft of a motor through a reducing device (e.g., a servo motor having a planetary gear reducing device connected thereto). The length of the first and second links 13 and 14 is set such that the dies 3a and 3b are brought into contact with each other, when the first and second links 13 and 14 have been expanded into a straight line. Such setting is performed by a die-thickness adjusting mechanism which is not shown.
A stopper 17 is fixed to the toggle support 4, so that when the first link 13 is rotated in the direction indicated by an arrow a to abut against the first link 13, a further rotation of the first link 13 is limited by the stopper 17 to retain the single-toggle mechanism at a dead-point position. At this dead-point position, the dies 3a and 3b are in contact with each other, so that the die-clamping is conducted by the clamping cylinder 8.
In the clamping cylinder 8, the piston 9 is mounted to the other end of the tie bar 7. However, the mounting of the piston 9 is not limited thereto, and for example, a hydraulic cylinder may be mounted to the other end of the tie bar 7. Further, the clamping cylinder 8 is mounted on the stationary platen 1, but alternatively, may be mounted on the toggle support 4.
The mold-opening and closing operation by the clamping device for the electric-powered injection molding machine will be described below.
When the motor is driven to rotate the first link 13 in the direction indicated by the arrow a through the shaft 16 to expand the first and second links 13 and 14 substantially into a straight line, the single-toggle mechanism 6 is brought into the dead-point position, thereby causing the first link 13 to be positioned and fixed. At this time, the dies 3a and 3b are brought into contact with each other, and the closing of the mold is carried out. The motor ensures that the first link 13 can be urged against the stopper 17 and placed at a stable position with a small torque.
After positioning of the first link 13 at the dead-point position of the single-toggle mechanism 6, a hydraulic pressure is supplied to the oil chambers which are defined adjacent the tie bars 7 and in the clamping cylinders 8 disposed at the other ends of the four tie bars 7, so that the tie bars 7 are pulled through the pistons 9 to generate a uniform and stable clamping force to bring the dies 3a and 3b into close contact with each other, thereby effecting the clamping of the mold.
In carrying out the opening of the mold, the oil within the oil chambers in the clamping cylinders 8 is drained and then the first link 13 of the single-toggle mechanism 6 is rotated in a counterclockwise direction (i.e., a direction opposite from the direction indicated by the arrow a).
In this way, the motor is used to move the movable platen 2, and the clamping cylinders 8 are used to generate the clamping force. Therefore, it is possible to use a small-sized motor.
In the above-described prior art clamping device for the electric-powered injection molding machine, however, roller bearings, which are not shown, are used to support the shaft 16 for connecting the motor with the first link 13 of the single-toggle mechanism 6 and to support the link pin 18 provided between the first and second links 13 and 14. The roller bearings have a load-carrying capacity and may be deviationally worn or fractured, when they receive a large clamping force.
Accordingly, it is an object of the present invention to provide a clamping device for an electric-powered injection molding machine, wherein the problems associated with the prior art clamping device for the electric-powered injection molding machine are overcome, whereby the durability of a toggle mechanism such as the above-described single-toggle mechanism can be enhanced, and an increased clamping force can be provided.